What is the most important factor in the decline of trees?

• A. Drought stress
• B. Ozone damage
• C. Insect infestation
• D. Toxic metals freed by acid deposition

Answer: (D)

The main idea here is how acid deposition alters soil chemistry to harm trees. When acidic rain or pollutants enter soils, the pH drops and toxic metals, especially aluminum, become more soluble. Aluminum ions injure root tips, disrupting water and nutrient uptake. At the same time, essential base nutrients like calcium, magnesium, and potassium are leached from the soil, weakening roots and reducing the tree’s overall nutrient balance and growth. This combination of direct root damage and nutrient deficiency makes trees more vulnerable to other stresses (drought, pests, and disease), so the decline can be widespread and persistent.

Drought stress and ozone damage can harm trees, but they don’t address the underlying soil chemistry changes caused by acid deposition that directly trigger many decline symptoms. Insect infestations can follow or coincide with stress but are not the primary pollutant-driven mechanism. The toxic metals released by acid deposition explain a mechanism that directly undermines tree health, making it the most important factor among the options.